A monotypic genus. Recent genetic studies of the captive population, and the indication of outbreeding depression have aroused suspicion that their may be more than one species in this genus (Vàsàrhelyi 2002).

Callimico goeldii occurs in the upper Amazon from the Rio Caquetá in Colombia, south through the Peruvian Amazon and the extreme western Amazon of Brazil into the Pando region of northern Bolivia (Hernández-Camacho and Barriga-Bonilla 1966; Hernández-Camacho and Cooper 1976; Hershkovitz 1977). Hershkovitz (1977) predicted that it should occur in the Ecuadorian Amazon, but it has not been found there to date. Despite its wide range, Callimico is notoriously patchy in its distribution and is evidently absent over a large part of it. In Colombia, it occurs from the base of the Cordillera Oriental of the Andes in the Department of Putumayo between the Ríos Putumayo and Caquetá east at least to the mouth of the Río Cahuinarí, a right bank affluent of the Caquetá. It is not known to occur in the Colombian trapezius (Hernández-Camacho and Cooper 1976; Defler 2003, 2004). In Peru, it is evidently limited largely to the eastern Amazon. Hershkovitz (1977) mapped numerous localities south of the Río Napo, along the lower and middle Río Ucayali and the Río Tapiche. The westernmost locality given by Hershkovitz (1977, map p.864) is on the Rio Marañon, but it is listed in the gazetteer as “Apaga (Rio), enters Río Putumayo from south at approximately 4º42'S, 77º10'W, P. Soini, April 1970, sight record”. The coordinates would seem to be right, but the description of the locality wrong, and the Rio Marañon is excluded from the distribution map of Aquino and Encarnación (1994a). These authors have Callimico only definitely occurring south of the lower Ucayali (from the mouth of the Rio Blanco), extending to both sides of the Ucayali at about 6ºS and south along the Andean foothills to the Río Pachitea and the Madre de Dios. It occurs in the Manu National Park (Aquino and Encarnación 1994a). From there it extends east into extreme northern Bolivia, north of the Río Tahuamanu (Buchanan-Smith et al. 2000; Christen and Geismann 1994). Christen and Geismann (1994) reported seeing Callimico south of the Río Nareuda, indicating it occurs south as far as the Rio Muyumanu. Buchanan-Smith et al. (2000) found no evidence of its occurrence south of the Tahuamanu-Nareuda. Callimico occurs in a small part of the south-west Brazilian Amazon in the state of Acre, through the Serra do Divisor south of the upper Rio Juruá to the Rio Gregório (state of Amazonas), to the Rio Iaco (above the Rio Acre) on the south (right) bank of the upper Purus, and into the Madeira basin along the Rio Abunã in the state of Rondônia (Hershkovitz 1977; Ferrari et al. 1999; Lopes and Rehg 2003).

Very localized and patchy distribution. Always rare and difficult to observe. Groups are generally widely separated, determined by the availability of the right mix/mosaic of habitats and food availability. Key habitat such as stream edge, bamboo forest and secondary growth patches are needed due to their predilection for certain species of fungus.

Rehg (2003, 2007) recorded 0.8-1.2 groups/km² in an 820-ha forest patch in Acre, Brazil. This is higher than density estimates recorded from Bolivia, and Rehg (2007) believed it might be related to habitat heterogeneity, including disturbance associated with tree falls.

Porter (2007) combined all group counts reported in the literature and calculated an average group size of 6.4 individuals, and a general population density of 5.6 individuals/km² where they occur. At one site in the far north-west of Bolivia, Porter (2007) estimated 37 individuals/km².

Occurs in forest mixed with bamboo, and in varillales (large stands of thin, straight trees). Izawa (1979), Buchanan-Smith (1991a), and Ferrari et al. (1999) have indicated that it is a habitat specialist concentrating on bamboo forests and disturbed forests (secondary growth) related to their tendency to spend much of their time in the lowest parts the forest understorey in dense vegetation. They can also be found in more open, mature forest (Christen and Geissmann 1994; Christen 1998, 1999). Porter (2007) quantified the time a group spent in different habitat types during the course of a year: primary forest with dense understorey 76%; bamboo forest 10%; secondary forest 3%. The group used bamboo forest more than the sympatric tamarins (S. fuscicollis and S. labiatus). Its preference for dense vegetation is related to its extraordinary inclusion of fungi in its diet and to its insect foraging behaviour. Bamboo, stream edge, and tree falls are where they are able to find the jelly fungi and bamboo fungi that they appreciate (Pook and Pook 1981; Porter and Christen 2002; Porte, 2007; Porter et al, 2007).

The diet of Callimico includes fruit, arthropods (especially orthopterans and stick insects), fungi, and exudates. Porter (2007; Porter and Christen 2002; see also Hanson et al. 2006) found that fungi were an essential and considerable component of the diet. Two types were eaten: jelly fungi (Auricularia, 3 species) and bamboo fungi (Ascopolyporous, 2 species). In some months (May-July, dry season) fungi accounted for 48-63% of the feeding records. Callimico forages for arthropods in the understorey and beneath the leaf litter on the forest floor. They are understorey specialists spending 84% of their time foraging below 5 m above the ground (saddleback tamarins tend to forage more a little higher, up to 10 m above the ground, and the moustached tamarins forage even higher up, mostly in the middle and lower canopy - above 10 m (Yoneda 1981; Buchanan-Smith 1999; Porter 2007). Callimico tends to eat more vertebrates (frogs and lizards) than the tamarins. For a review see Heltne et al. (1981).

In her long-term study of Callimico, Porter found that one group used an area of 114 ha, covering the entire home ranges of six tamarin groups (Porter 2007; Porter et al. 2007). Rehg (2003, 2007) on the other found her Callimico group using a range of about 59 ha; similar in size to the Saguinus labiatus group which they travelled with for about 67% of the time. Group size 7 (six adults and subadults and an infant). Callimico groups can be as large as 12 (Encarnación and Heymann 1998) and unlike other callitrichids can have more than one breeding female (Masataka 1981a, 1981b). Copulations occur as soon as 10 days after the female gives birth indicating post-partum oestrus. They produce single offspring. Other members of the group help carry the young (communal breeding), with the first transfer from the mother a few days (4-11) after birth.

Although there are probably no major threats at present, the species is at risk of becoming threatened very quickly depending on proposed development projects, and logging in areas where it is known to occur.

Bamboo often reforests areas that have been cleared for agriculture or pasture. Studies are needed to determine Callimico's degree of tolerance for habitat disturbance in its range (Porter 2007).